1. RF detection
Passive RF detection is the basis for effective countermeasures against UAS. RF sensors provide detection capabilities by matching drone communication protocols to known drone RF signatures. Commercial and consumer-grade UAS use a variety of protocols, some of which are proprietary. RF-based detection systems that simply scan frequency bands commonly used by drones, such as those that use Wi-Fi scanners or “sniffers,” have an extremely high rate of false alarms.
RF sensors are generally “passive” and do not broadcast or emit signals. This enables RF countermeasure UAS to operate without interfering with the network or other communications within the operational area. Unlike passive RF systems are those that actively use the protocol to manipulate or attempt to “hack” the drone.
In addition to the passive advantages of RF systems, there are often other desirable characteristics of using RF to counter UAS. Key capabilities depend on the operating scenario and environment, but when evaluating RF solutions, the following factors need to be considered:
• Large, scalable RF signature library or detection engine, providing high detection probability and low false alarm rate
• Ability to flag or filter false alarms to optimize and improve performance over time
• Azimuth and pitch coverage optimization for drone or drone detection
• RF direction finding capability of the UAV and its controller
Considering the cost per sensor, RF sensors typically provide a longer detection range than radar systems. RF-based direction finding capabilities can also provide tracking capabilities similar to radar systems.
Radio frequency can be a suitable technology for drone detection, as evidenced by innovative solutions, for example, wearable, handheld and vehicle-mounted products that offer the ability to counter drones in field environments and “mobile” operations .
As with all approaches, any effective system needs to overcome some challenges. Multipath exists in most practical environments and can significantly reduce the accuracy of RF systems. This is due to signal reflections, where the system receives signals from multiple directions at the same time. Any effective system should be able to determine the orientation of these signals with high accuracy, despite the presence of multipath.
Figure 6: RF triangulation of drone position
How will RF drone detection (and countermeasures) technology evolve?
With the advent of LTE-controlled drones, RF sensor technology must continue to evolve.
Another common problem is the performance of the RF sensors of “autonomous” drones. While many so-called autonomous drones still emit telemetry and video data, making them detectable by RF sensors, the kind that carry an SD card (or similar) and use a camera to navigate or an inertial navigation system (INS) Machines are more difficult to detect and therefore rely on other sensors to receive RF. UAV radio frequency communication is roughly divided into:
• Remote control frequency bands for hobbyist drones that cannot be used for anything other than low baud rate telemetry control.
• ISM (Scientific or Industrial) bands, hobbyists are largely unlicensed and regulated in terms of application, output power and spectral purity.
• Commercial drones operating compliantly in frequency bands allocated by their respective national civil regulators
• Military drones, not regulated by civilian regulators using traditional military communications bands, details unclassified.
Any radio frequency control operation other than the first three categories is considered illegal operation and violates the radio communication laws of the respective countries.
Third-party transceivers, modems, and up/down converters available in certain countries may be illegally imported and used, and capable drone manufacturers are kept abreast of these developments as they emerge.
The technological capabilities and innovation capabilities of counter drones are increasing exponentially, providing effective drone and remote control detection and countermeasure products to meet and challenge advanced drone technology when necessary.
Radar can effectively track the trajectory of the target, in addition to tracking drones, there are many other applications. For counter-drone applications, the key is to use a radar with sufficient resolution to detect small drones, such as DJI’s drone platforms, at typical distances (usually 1km or more). Many radars, such as low-band pulse radars, have been designed to detect large metallic objects such as aircraft and helicopters, but are not suitable for detecting objects with small RCS and low flying, such as drones in Groups 1 and 2.
Additional considerations when evaluating counter-drone radar options include:
Dealing with ground clutter: Ground clutter can interfere with radar detection, and objects such as trees and buildings are prone to false alarms because they look a lot like drone blades on radar. Advanced radar systems will apply various techniques to reduce this effect.
Azimuth Coverage: Azimuth is the horizontal coverage angle of the radar. A typical range for a system is 90 degrees to 360 degrees. In less than 360 degrees, multiple radars can be used to provide coverage of large angles or even entire ranges.
Pitch angle: The pitch angle of the radar is easily overlooked. The pitch angle of many radars on the market is very narrow, between 10-30 degrees, which leads to a huge blind spot in the pitch coverage. While 90-degree vertical coverage is generally not required, a 40-80-degree detection range is considered ideal.
2D vs 3D: 3D radar has several advantages over 2D radar, most notably, it provides altitude information for drones. The ability to reduce clutter by filtering objects above a certain height helps eliminate ground-based false alarms.
Frequency bands: Anti-drone radar bands include X-band (also commonly used on ships), K-band (originally used for autonomous vehicles, but suitable for anti-drone), Ku-band, and S-band (common to military deployments). The use of these bands also has an impact on the size of the radar, for example, K-band radars typically have a smaller form factor.
Moving/Fixed Panels: Depending on the use case, fewer moving parts may be best as this reduces the chance of wear and damage. Some radars can be used in both “staring” and rotating modes, with the staring mode providing better performance but with a reduced angle of coverage.
UAV detection radar can use different technical methods. Radars used in counter-drone solutions use one of three technologies: pulsed (active), continuous wave (active), and passive modes. Each method has different characteristics, each with its own advantages and disadvantages:
Active Radar – Pulse: Sends a very short but high power pulse and waits for a reflected echo from the target. The reduction of the transmit pulse and echo receive time windows can affect performance. The shorter the pulse width, the higher the range resolution. Therefore, pulsed radars are generally designed for long distances.
Active Radar – Continuous Wave: The radar continuously transmits an RF signal and simultaneously receives the reflected echo. Due to the influence of the Doppler effect, the receiver can determine the speed and trajectory of the object by measuring the frequency shift. Continuous wave systems cannot make distance measurements without including a timing reference in the transmit signal.
Figure 7: Micro-Doppler radar to detect drones
Passive Radar: Detects the presence of objects in the receiving area using existing ambient broadcast, communications or radio navigation signals. The system transmitter and receiver are located in different locations and the user can only control the receiver. Potential transmit signals that can be used for UAS detection include FM, DVB, GSM, GNSS or WIFI. This approach is attractive to end-users who prefer to use non-transmitting equipment so that no noticeable features are created during operation.